The present invention is directed to ferroelectric and electrostrictive ceramic materials, especially those being lead-free and showing very strong remnant polarization and massive pure electrostriction, especially in (Sr,Bi,Na)TiO3 solid solutions. The composition of the present invention is further directed to a solid state solution of bismuth-sodium titanate and bismuth-strontium titanate.
Ferroelectric materials show that the permanent electric dipole moment of materials can be reoriented by the application of an external electric field. The piezoelectric effect can be described in a simple way: electricity is generated when mechanical pressure is applied. Inversely, by applying an electric field to a piezoelement, a mechanical deformations result. This is referred to as the inverse piezoelectric effect. The inverse piezoelectric effect has sometimes been confused with the electrostrictive effect which occurs in solid dielectrics. The two effects differ in two important respects. The piezoelectric strain is proportional to the electric-field intensity and changes sign with it, whereas the electrostrictive strain is proportional to the square of the field intensity and therefore independent of its direction. Further, the piezoelectric strain is usually larger by several orders of magnitude than the electrostrictive strain, and the electrostrictive effect occurs simultaneously with the piezoelectric effect but may be ignored for practical purposes in many cases.
In some materials, the electrostrictive effect is significant and can be used for practical applications. For example, Pb(Mn1/3Nb2/3)O3 (PMN) and its solid solutions Pb(Mn1/3Nb2/3)O3—PbTiO3 (PMN-PT) exhibit a high electrostrictive strain level of ˜0.1% at 70 kV/cm, with the electrostrictive coefficient Q11=˜0.02 m4C−2. PMN, often, is referred to as an “Electrostrictive Ceramic”. Compared with the piezoelectric effect, the electrostrictive effect has several unique advantages, such as less or no hysteretic loss up to high frequencies, being temperature-stable, and exhibiting a fast response time.
Currently, thousand tons of lead-containing ferroelectric/electrostrictive materials, such as PbZrO3—PbTiO3 (PZT), Pb(Mg1/3Nb2/3)O3—PbTiO3 (PMN-PT), and Pb(Zn1/3Nb2/3)O3—PbTiO3 (PZN-PT), are produced every year for a large range of applications. Due to the toxicity of lead, alternative lead-free materials are highly desirable for environmental reasons. New environmental legislation enacted by the European Union (EU) will take effect on Jul. 1, 2006 (Directive 2002/95/EC of the European Parliament and of the council of 27 Jan. 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment). The Restrictions on Hazardous Substances directive limits the use of lead, cadmium, mercury, hexavalent chromium and two brominated fire retardants. It is expected that USA and Japan will also have similar restrictions in near future. Therefore, substitutes for the toxic lead containing materials are highly desirable for electronic industry.
In the last two decades, great effort has been made to search for high performance lead-free materials. For instance, (Bi0.5Na0.5)TiO3-based, Bi4Ti3O12-based, SrBi2Ta2O9-based, and BaTiO3-based systems have been extensively studied. However, the remnant polarization and electrostrictive properties of these lead-free materials are still far below those of the currently used lead-containing materials. Some examples are listed are barium titanate BaTiO3 materials and bismuth sodium titanate (Bi0.5Na0.5)TiO3 materials
Bismuth sodium titanate (Bi1/2Na1/2)TiO3 (BNT) is more similar to PZT, with a Tc at ˜230° C., and room temperature dielectric constant (∈=2500˜6000). Some modified BNT ceramics manifest reasonable ferroelectric/piezoelectric properties, which make the material promising as a substitute for PZT. Since the late 1980s, efforts have been made to study the possibility of substitution of lead-free BNT for PZT. Simple perovskite compounds, like BaTiO3, PbTiO3, SrTiO3, etc., were used to modify the ferroelectric/piezoelectric properties of BNT. Nagata and Takenaka (“Effect of substitution on electrical properties of (Bi1/2Na1/2)TiO3-based lead-free ferroelectrics”; Proc. of 12th IEEE Int. Symp. on Applications of Ferroelectrics (ed. Streiffer, S. K., Gibbons, B. J. and Trurumi, T.); vol. I, pp 45-51; Jul. 30-Aug. 2, 2000; Honolulu, Hi., USA.), for example, obtained the highest remnant polarization Pr value of 33.7 μC/cm2 reported for (Bi0.5Na0.5)TiO3 systems, which appears to be one of the best result to date published in the literature for BNT-based materials.
Very high strain (up to ˜1%) has been observed in a series of ferroelectric/piezoelectric/electrostrictive materials (single crystals and ceramics), such as, tetragonal BaTiO3 single crystals, and (Bi1/2Na1/2)TiO3-based materials. However, the strain versus electric field profile exhibits large hysteresis at a reasonable frequency (for example, at 1 Hz) due to the piezoelectric effect. The material does not exhibits a pure electrostrictive effect. This becomes a critical disadvantage when the displacement of actuators needs to be precisely controlled.
A number of patents and patent applications have disclosed attempts at establishing a non-lead dielectric ceramic composition. For example, U.S. Pat. No. 5,637,542 to Takenaka teaches a binary system, solid solution represented as (1-x) BNT-xNN, wherein BNT is (Bi1/2Na1/2)TiO3 and NN is NaNbO3 as constitute to the solid solution. U.S. Pat. No. 6,004,474 to Takenaka et al. teaches a lead-free piezoelectric material represented by the formula x[Bi1/2NA1]TiO3-y[MeNbO3]-(z/2)[Bi2O3.Sc2O3], where Me is K or Na. Other lead-free piezoelectric ceramic compositions using BNT are taught in U.S. Pat. No. 6,514,427 to Nishida et al.; U.S. Pat. No. 6,531,070 to Yamaguchi et al.; U.S. Pat. No. 6,258,291 to Kimura et al.; US Patent Application 2004/0127344 to Sato et al.; and US Patent Application 2003/0001131 to Takase et al. Chiang et al in US Patent Application No. 2002/0036282 teaches a perovskite compound which functions as an electromechanically active material and may possess electrostrictive or piezoelectric characteristics. The compound is an alkali bismuth perovskite composition in which lead can be a part.